Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers are used. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little “wet strength”. Wet strength of ordinary paper is only about 5% of its dry strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water. See U.S. Pat. No. 5,585,456. To overcome this disadvantage, various methods of treating paper products have been employed.
Wet strength resins applied to paper are either of the “permanent” type or the “temporary” type, which are defined by how long the paper retains its wet strength after immersion in water. While wet strength retention is a desirable characteristic in packaging materials, it presents a disposal problem because paper products having such characteristics are degradable only under undesirably severe conditions. Some resins impart temporary wet strength and are suitable for sanitary or disposable paper uses; however, these resins often suffer from one or more drawbacks. For example, the wet strength of the resins is generally of a low magnitude (about one-half of the level achievable for permanent-type resins), the resins can be easily attacked by mold and slime, and/or the resins can only be prepared as dilute solutions.
Conventional resins, which are able to provide permanent wet strength to paper, typically are obtained by modifying polyamidoamine polymers such as A, with epichlorohydrin (B) (“epi”) to form polyamidoamine (PAE)-epichlorohydrin resin.

Conventional resin syntheses capitalize on the difunctional nature of epichlorohydrin to use the epoxy and chlorine groups for both cross-linking and generation of quaternary nitrogen sites. In these conventional resins, the asymmetric functionality of epichlorohydrin leads to ring opening upon reaction of its epoxy group with secondary amines, followed by the pendant chlorohydrin moiety either intra-molecularly cyclizing to generate azetidinium functionality or inter-molecularly (cross-linking) with another polyamidoamine molecule. Thus, the first step of reacting polyamidoamine prepolymer A with epi B occurs with ring-opening of the epoxy group by secondary amine groups of the prepolymer backbone at relatively low temperature. New functionalized polymer C having chlorohydrin pendant groups is generated, and this process typically results in little or no significant change in the prepolymer molecular weight.

The second step involves two competing reactions of the pendant chlorohydrin groups: 1) an intramolecular cyclization which generates a cationic azetidinium chloride functionality, in which no increase in molecular weight is observed; and 2) an intermolecular alkylation reaction to cross-link the polymer, which significantly increases its molecular weight. The results of both reactions are illustrated in the PAE-epichlorohydrin resin structure D. In practice, the alkylation of epichlorohydrin, the intra-molecular cyclization and the cross-linking reactions are occurring simultaneously, but at different rates.

The finished wet strength polymer product contains a small amount of residual pendant chlorohydrin as illustrated in structure D, and a 3-carbon cross-linked group with 2-hydroxyl functionality, with a fairly large amount of quaternary azetidinium chloride functionality. The product also can contain substantial amounts of the epichlorohydrin hydrolysis products 1,3-DCP, and 3-CPD.

The relative rates of the three main reactions in this conventional method, namely the pendant chlorohydrin formation (ring opening), cyclization to azetidinium ion groups (cationization), and cross-linking (intermolecular alkylation), are approximately 140:4:1, respectively, when carried out at room temperature. Therefore, the pendant chlorohydrin groups form very quickly from ring opening reaction of the epichlorohydrin epoxide and the secondary amine in the prepolymer. This first step is performed at lower temperature (for example, around 25° C. to 30° C.).
In the second step, the chlorohydrin groups relatively slowly cyclize to form cationic azetidinium groups. Even more slowly, cross-linking occurs, for example, by: 1) a tertiary amine, for example, of a chlorohydrin pendent group reacting with moiety secondary amine; and/or 2) intermolecular alkylation of a tertiary amine with a pendant chlorohydrin moiety.
In order to maintain practical utility for minimum reaction cycle times, the conventional manufacturing process typically requires that the reaction mixture be heated to increase the reaction rates, for example to about 60° C. to about 70° C. Usually, reactions are also carried out at high solids content in order to maximize reactor throughput and provide finished wet strength resins at the highest solids possible to minimize shipping costs. High concentration favors the slower, inter-molecular reaction. Under these high temperature and high concentration conditions, the reaction rates between intramolecular cyclization and cross-linking become competitive. Thus, one problem encountered in the conventional manufacturing process is that the cross-linking reaction rate becomes fast enough that the desired viscosity end-point (molecular weight) is achieved at the expense of azetidinium ion group formation. If the reaction was allowed to continue beyond the desired viscosity end-point in order to generate higher levels of azetidinium groups, the reaction mixture would likely gel and form a solid mass.
Since both high azetidinium group content and high molecular weights are useful for maximum wet strength efficiency of PAE resins, azetidinium group formation and cross-linking desirably are maximized without gelling the product or providing a product that gels during storage. These conditions, coupled with the desire for high solids to minimize shipping costs, have been limiting aspects of the formation of higher efficiency wet strength resin products.
There is a need, therefore, for improved strengthening resins, e.g., for imparting appropriate levels of wet strength to paper products, and methods for making and using same.